The reaction proceeds following a ping-pong mechanism forming a covalent intermediate between an active site serine and the L-phenylalanine residue [2]. The protein, found in the bacterium Streptomyces noursei, also forms cyclo(L-phenylalanyl-L-phenylalanyl), cyclo(L-methionyl-L-phenylalanyl), cyclo(L-phenylalanyl-L-tyrosyl) and cyclo(L-methionyl-L-tyrosyl) [1].